Two observations. First of all, "no-preparation" Thaumaturgy is discussed elsewhere. For example:
"If the complexity of the spell is equal to or
less than your Lore, assume you have everything
you need to cast the spell and you
require no additional effort for preparation." (YS262)
However, it's not entirely clear to me that meeting this requirement makes the Thaumaturgy able to be cast at Evocation speed. Consider:
"The first is a matter of time. Thaumaturgy
is a slow art, with the fastest of spells taking a
minute or more" (YS261)
This strikes me as a lot more time than it takes to fling a fireball. My interpretation (note emphasis) of this is as follows: If you meet the complexity requirement with Lore alone, then you have everything you need on hand. You need not skip scenes, gather components, perform research, tap arcane power sources, etc. as preparation for the spell. But you do have to pull components out of your pockets, draw a circle, clear your thoughts, and such.
So how long does that take? Well, the comment above indicates it takes about a minute or so, and I think that seems in line with some of Dresden's simpler rituals. I don't see any hard rules on how long turns are in DFRPG, beyond this:
"Exchange (conflict only): The amount of time
it takes in a conflict for everyone to take one
action and defend against any actions that are
directed against them—usually not longer
than a few minutes." (YS314)
So it takes at least an exchange worth of time to cast a ritual. But to differentiate rituals from evocations, I'd argue that it should take a full exchange. So you set up the ritual and start casting it during one exchange, then you make the control roll (or the first control roll as the case may be) in the next exchange. Once you've controlled all of the power, the spell goes off. In the meantime, you are concentrating, and therefore largely defenseless -- so you might want some friends to fend off the nasties.
Well, that's my take. YMMV.